The present invention is directed to a process and the product of that process wherein a quenching means is employed that is controllably variable and provides a metal alloy with increased strength imparted to the alloy during finishing of the alloy manufacture. The quenching means is through the use of a liquid after the alloy has been worked and/or heat treated, followed by an air quench.
It is well known by those skilled in the metal alloy art that working and/or heating an alloy, and subsequently following that worked and/or heated alloy with a quenching step, may impart enhanced properties. A problem in the manufacture of thinner sheet alloy is that quenching a thin sheet alloy can cause multi-directional thermal distortion in the final product. What was a flat unwavering product becomes a bent, uneven, and/or physically distorted sheet alloy. While this problem is particularly troublesome in the thin sheet alloy art, it is also a problem in the forged and cast alloy art.
Heat treatable alloys contain soluble alloying constituents in amounts which exceed their room temperature solubility limits. The solution heat treating process involves heating the alloy to a sufficient temperature to permit desired constituents to go into solid solution. The resultant supersaturated solid solution can be sustained at room temperature if cooled or quenched rapidly enough to prevent precipitation. Constituents in an alloy system mean those minor metal components in the alloy that can have a significant impact on properties if present in the right place in the alloy and in the right amount. Room temperature mechanical and physical properties can depend on the extent to which the alloy constituents remain in solid solution.
Elevated temperature quenching can also result in undesirable physical distortion of the metal strand due to thermal contraction of the alloy. A strand can be sheet, slab, extrusion or other worked and/or heat treated metal alloy based in an iron, magnesium, titanium, and/or aluminum system, preferably aluminum alloy. In a continuous or semi-continuous process, the magnitude of distortion is proportional to the rate the strand is cooled. Achieving desirable mechanical properties by solution heat treatment followed by a quenching step then involves competing interests between enhanced mechanical properties and thermally induced physical distortion.
The present invention is useful for the high speed manufacture of metal alloys with higher strength values. These alloys may then be used to make articles of manufacture.
A process for the manufacture of metal alloys wherein a controllably variable liquid quenching means is used to rapidly cool the metal alloy at or above the Leidenfrost temperature prior to and in combination with an air quenching means and alter heat treatment whereby the metal alloy is quenched without metal alloy distortion providing a metal alloy with superior tensile strength properties. The process is comprised of finishing metal alloy on a horizontal bed, translation of said metal alloy sequentially through a solution heat treating furnace, a liquid quench chamber with a single and/or a plurality of controllably variable spray orifices, followed by a gas quench chamber. The spray orifices create a spray or mist in order to wet the metal alloy. While the preferred finishing bed is on a horizontal translation, the underlying invention is not planar dependent in the horizontal direction and embraces all directional translations, such as vertically. The invention hereof is specifically directed to the effective length of the liquid quench chamber, the enablement of which is the use of multiple zones of liquid flow through orifices. By zones it is meant an area within the liquid quench chamber whereby a plurality of separate orifices may be individually controlled within the liquid quench chamber. The number of zones and/or the time spent in a zone for a metal alloy, preferably aluminum alloy, may be controlled in consideration of the composition of the alloy being processed, the size of the strand, the speed of the translation and the liquid treatment or application means operating parameters, such as orifice type, pressure, the physical properties of the liquid, and flow rate. The gas is preferably air; however, any inert or benign gas will be sufficient to cool the strand.
The desired cooling rate is also dependent upon the kinetics of precipitation. The kinetic rate is temperature and solute dependent. The kinetic rate constant is temperature dependent. The rate constant is nearly zero at high and low temperatures. Accordingly, the loss of strength associated with a loss of solute from solution approaches zero at these high and low temperatures. For temperatures in a certain regime, near what is called the critical temperature, the kinetic constant is of sufficient magnitude to effect losses of solute from solution and lead to decreased strength potential in the alloy. The importance of the quench is to minimize the loss of solute from the solution. Therefore, an understanding of the cooling rates, particularly in the critical temperature range, provides insight into how the cooling rate can be maximized. To determine how to vary the controllable or tunable quench, it is important to know the temperature regime near the critical temperature. The rapid cooling of metals leads to undesirable residual stresses and distortion in the metal alloy. Increased cooling rates are accompanied by increases in thermally induced stresses. If these stresses occur at elevated strand temperatures, they can become permanent plastic deformations in the alloy. Thermally induced residual stress and physical distortion can be minimized by reducing product cooling rates. Distortion is also influenced by the thickness of the metal alloy which may be within a range of 0.01 to 8 inches thick. The thicker or metal alloy slabs will be less sensitive to the distortion than the thinner strips or slabs.
The challenge is to provide cooling rates which are sufficiently high to retain solute in solid solution but not in excess of those which lead to permanent plastic distortion. The magic of the present invention is the advantage that is taken by spraying liquid, preferably water, onto the surfaces of the metal alloy at elevated temperatures. Other liquids, such as water/glycol combinations or other organic liquids such as alcohols may be used if they have the appropriate sprayability and viscosity to exit the orifices and quench the strand. Additionally, dissolved gases such as CO2 may be used in the liquid coolant. The liquid quench cools the metal through the critical temperature at a rate that can be controllably varied from each metal alloy composition and can increase or at least maintain the strength potential of the alloy without physical distortion. Heretofore, the air quench was believed to provide the highest strength characteristics with minimum metal alloy distortion.
For the cooling of continuous and/or semi-continuous strands or metal alloy, the present invention makes use of an array of zoned water flow-through spray orifices in order to control the time which the strand remains above the critical temperature. While the inventors hereof do not want to be held to any particular theory of operability of the present invention, some theory may aid in the understanding of these teachings for those skilled in this art. The heat flux of a heat treated strand can undergo a rapid order of magnitude increase as the alloy is cooled below the temperature TL known as the Leidenfrost temperature, which may be between about 350xc2x0 to 700xc2x0 F., depending upon the metal alloy. Above TL, the alloy is blanketed by a vapor film which limits the heat removal. When the temperature drops below TL, the vapor film breaks down, the surface is wetted by the water droplets, and the heat flux can increase dramatically. TL is functionally and operatively related to the specific spray orifice, flow rate, physical and chemical properties of the liquid, and pressure used to apply the liquid, the liquid preferably being water. TL is also related to the momentum of the droplets which contact the strand surface. Droplet momentum can be increased by increasing droplet size or increasing its impact velocity. High momentum droplets more easily penetrate the vapor film, thereby increasing TL. For a given liquid, spray orifice or nozzle configuration, flow rate, and pressure, TL is constant.
Important for the operability of the present invention is the operation of the water quench mechanism in order to maximize the time the strand remains above TL. Maintaining the temperature above TL delays the rapid increase in the cooling rate until the strand is sufficiently rigid to resist plastic distortion. Contemplated within the reach of the present invention is that for certain alloys and/or strand sizes, the desired mechanical properties can be achieved by operating the water quench exclusively above TL. For certain alloys and strand sizes, no liquid cooling is required, and sufficient strengths can be achieved by gas cooling.
Aluminum alloys, especially those heat treatable alloys of the 6XXX, 2XXX, and 7XXX series known also as the 6000, 2000, and 7000 series aluminum alloys, preferably Aluminum Association registered alloys, 2008, 6022, and 6111, are particularly advantaged by the use of liquid quenching followed by a gas, preferably air, quench.